1. Field of the Invention
The present invention relates generally to a defibrillator and, more particularly, to a defibrillator that provides variable waveforms.
2. Related Art
A defibrillator is a device used to administer a high intensity electrical shock through two or more electrodes, commonly referred to as "paddles" or "pads," to the chest of a patient in cardiac arrest. A selected, discrete quantity of energy is typically stored in a charge-storage device (e.g., a capacitor) and is then electrically discharged into the patient through the paddle circuit.
Defibrillation is not a procedure with a certain and successful outcome. Rather, the probability of successful defibrillation depends on the condition of the patient and on various defibrillation discharge parameters, such as the intensity and shape of the defibrillation waveform. The term "intensity" as used herein refers to the level of the electrical discharge, which may be measured by the energy stored in the defibrillator, energy delivered to the patient, peak or average current flowing through the patient, electrical charge or integrated electron flow through the patient, and various other measures.
In order to practice defibrillation successfully and safely, it is important that the defibrillator be capable of providing an electrical discharge having an optimal combination of discharge parameters. For example, if the selected discharge intensity level is too low, defibrillation will not be successful and must be repeated at a higher intensity level until the patient is defibrillated. However, repeated defibrillation discharges at increasing intensity levels are more likely to cause damage to the heart. Also, repeated discharges cause the patient to remain in ventricular fibrillation for a longer time. This delay may cause the patient's condition to deteriorate, as metabolic imbalance and hypoxia develop, which, in turn, make the patient harder to defibrillate and reduce the prospect of successful recovery.
Moreover, optimal intensity generally varies in accordance with the pulse duration. That is, desirable values of peak or average current delivered to the patient, charge or energy delivered to the patient, or other measures of intensity, vary with pulse duration. More specific information in this regard is provided in U.S. Pat. No. 5,111,813 to Charbonnier, et al.
More generally, the shape of the pulse may influence the outcome of the defibrillation attempt. The word "shape" as used broadly includes such parameters as amplitude, duration, polarity, number of pulse phases, or form (such as rectilinear, sinusoidal, etc.). As applied here to the electrotherapeutic discharges generated by defibrillation devices, the word "shape" is synonymous with the word "waveform."
A wide variety of defibrillation waveforms are known. Some defibrillators employ monophasic (single polarity) current or voltage pulses. Others employ biphasic (both positive and negative polarity) pulses. Either monophasic or biphasic pulses may have a variety of forms, such as sinusoidal, damped-sinusoidal, exponential, truncated-exponential, rectilinear, square, constant "tilt" (a measure of the difference between the start and end voltage, often expressed as the difference between the initial and final voltages, divided by the initial voltage), combinations of such forms, and so on. Moreover, the choice of which waveform to employ may depend not only on the condition or electrical characteristics of the patient, but also on whether the defibrillator is implanted in the patient or is applied externally to the patient. If the defibrillator is implanted, the patient's unique electrical characteristics and overall physiology may be investigated and the waveform tailored to that particular patient's needs. External defibrillators, in contrast, are intended to be applied to numbers of patients that have generally varying physiological characteristics. Moreover, the same patient may require different waveforms for optimal operation depending, for example, on the contact that is achieved between the paddles and the patient. Thus, external defibrillators may be designed for optimal use on an average patient. Alternatively, they may be designed so that they are capable of providing a variety of waveforms depending on an evaluation of the patient's physiology, the electrical connection achieved between the paddles and the patient, new knowledge about the operation and affect of electrotherapeutic discharges, or other factors.
In addition, the electrical requirements of defibrillators, particularly external ones, impose certain constraints on the type of circuitry, and components, that may practicably be used. In general, because of the indirect nature of the electrical connection (i.e., through skin and other tissue), external defibrillators operate at higher voltages and/or currents than implanted defibrillators. Thus, for example, the sizes of capacitors used as charge-storage devices typically are greater for external defibrillators as compared to internal ones, and often are more expensive. Also, other electronic components, such as solid state switches, diodes, inductors, resistors, and so on, must be capable of reliably operating in the relatively higher ranges of voltage and/or current. Moreover, because external defibrillators often are portable (because time is of the essence in applying the electrotherapeutic discharge), the weight of the device may also be an important factor. The weight of the defibrillator generally increases as the "size" of electrical components (i.e., their ability to generate, and operate under, higher voltages and/or currents) increases.
Therefore, it may be desirable that a defibrillator be capable of generating a variety of waveforms for electrotherapy (although such flexibility may not be important if, for example, a particular waveform is thought to be generally optimal). Moreover, it generally is desirable that the defibrillator employ components that are capable of operating at high voltages and/or currents, are not expensive, and are not heavy. Also, it is important that the defibrillator be capable of providing a complete electrotherapeutic discharge after only one charging of the charge storage device. If a second charging is required to complete the electrotherapy, potentially injurious delay results, the initial discharge may not have the intended therapeutic effect, and the repeated applications may injure the patient. Also, in an external defibrillator, the electrical characteristics of the paddle-patient connection may vary between applications.
Various defibrillation circuits are known that attempt to address one or more of these problems. For example, U.S. Pat. No. 5,749,905 to Gliner, et al., seeks to automatically vary an externally applied, biphasic, waveform by varying the duration of particular phases of the waveform depending on the electrical characteristics of the patient. This waveform manipulation is said to allow the production of a smaller, more efficient, and less expensive defibrillator. U.S. Pat. No. 5,769,872 to Lopin, et al., is directed to a defibrillation circuit that controls resistance between the charge-storage device and the paddles in order to manipulate a current waveform. A device is described in U.S. Pat. No. 5,222,492 to Morgan, et al., that uses pulse-width modulation to control the magnitude of a current administered to a patient.
Significantly, however, these and other known circuit designs do not optimally balance the need for variable waveforms with the need for smaller, lighter, and less expensive circuit components. For example, the capability of varying the waveform may be achieved in these devices by providing a high initial charge at the charge-storage device. As the waveform is generated, the initial charge dissipates and, in these known defibrillators, the amplitude of voltages (or currents) available to shape the waveform also generally declines. Pulses of longer duration, and/or higher amplitude during particular phases, thus may be achieved, but at the cost of providing a higher initial charge. As noted, this higher initial charge increases the weight and expense of the charge-storage device, and imposes greater voltage and/or current stresses on other electronic components in the defibrillation circuit.
Therefore, what is needed is a defibrillation device and method that enable a variable defibrillation waveform to be applied to a patient while reducing the voltage and/or current stresses on components.